The TI-99/8 was to be TI's ultimate answer to critics of the TI-99/4A and to the companies fighting for space within the home computer market. Unfortunately, it was not meant to be. Only 150 prototypes were built before TI withdrew from the market, using two very different versions of the motherboard. About half of the finished machines were of each motherboard type, further restricting utility of some of the surviving peripheral devices.
The TI-99/8 was intended to be a major improvement over the TI-99/4A. It met that mark in all but one respect: the TMS-9118 video processor used on both versions of the TI-99/8 has a feature set almost identical to that of the TMS-9918A used in its predecessor.
There is 64KB of random access memory standard on both versions of the TI-99/8 motherboard, with the possibility of extending that to 15MB using memory cards in the Peripheral Expansion Box (PEB). 220 KB of System ROMs contains the code for the operating system, the HexBus I/O interface, TI Extended BASIC II, and the core support routines for the UCSD p-System. Most of the ROMs were actually GROMs, a specialized medium-speed memory chip developed by TI with its own 13-bit address bus. GROMs are treated as devices by the operating system. The majority of surviving machines do not have fully-functional versions of the UCSD p-System GROMs (or don't have them at all).
There is no need for an external speech synthesizer for the TI-99/8, as it is included on both versions of the motherboard.
The TI-99/8 uses a TMS-9995 microprocessor clocked at 10 MHz. The main screen has an option to slow this down to the same speed as a TI-99/4A so that arcade game software written for the older machine remains playable.
The source code for the TI-99/8 is available and has been assembled to create an emulated version of the computer operating under MESS. The commented code is located on the WHTech FTP site, along with documents containing the specifications for interfacing with the Hex-Bus peripherals and the source code for the TI-99/8 implementation of the Hex-Bus Interface. The website for the Amarillo User's Group also has documents outlining the known bugs in the code available to us. Most of these bugs are within Extended BASIC II.
Two different avenues to peripheral expansion exist: cards for the PEB and/or HexBus peripherals. Bus speed dictates that memory expansion devices must connect through the PEB, as the HexBus does not have sufficient throughput to support it. Note that many of the peripheral expansion cards designed for the TI-99/4A will also work with the TI-99/8. Follow the link for descriptions of the various TI-99/8 Peripherals.
Very little software designed specifically for the TI-99/8 was produced. Most of the cartridge software for the TI-99/4A works, with some notable exceptions. The Extended BASIC cartridge does not work at all, because it conflicts with the built-in Extended BASIC II. The Editor Assembler cartridge fails when it checks for the 32K Memory Expansion card as part of its startup routine. There are problems with the Terminal Emulator II cartridge, in that only the terminal emulation functions of the cartridge work. Third-party cartridges also have problems, as the V3.0 Operating System installed in the TI-99/8 expects at least one GROM in the cartridge and won't execute those that don't have one present.
Extended BASIC II
Extended BASIC II adds many useful functions and commands to the Extended BASIC programmer's tool set. It also recognizes a much larger memory space. A standard TI-99/8 console shows approximately 60K of free space for programs, and the addition of a 128K Memory Expansion increases that to over 170K.
UCSD Pascal V4.11
The version of the p-System delivered with the TI-99/8 is an incremental improvement over Version 4.0 (the version installed on the p-Code card for the TI-99/4A). The system allows programs up to approximately 62K in size, and it is designed to utilize the 128K Memory expansion space for program variable storage and stack space when present (64K each).
Almost all surviving copies of TI-99/8 documentation are preliminary versions with significant editor's comments in the margins. The so-called "Blue Book" is the most common of these, containing most of the documentation intended for owners of the TI-99/8. At least one spiral-bound User's Guide printed as a mock-up of the final version also exists (though still bearing the preliminary notice on the front cover).
The primary system manual for the TI-99/8 is an internal preliminary document informally known as the "Blue Book." It contains descriptions of the computer, the built-in portions of the UCSD p-system, and a complete description of the commands included in TI Extended BASIC II. These manuals can be found online at WHTech.
A set of hand-drawn schematics describing the TI-99/8 is available as a PDF file on the WHTech FTP site. They are not complete (likely missing only the last page), as the final page of the PDF document only includes the signals present on slightly more than half of the expansion port pins. The drawing also numbers them incorrectly. The schematics are for a Revision 1 motherboard, which uses a 50-pin Centronics interface. The pins are numbered 1-25 across the top row and 26-50 on the bottom row. The individual who made this diagram numbered them with odd numbers on the top row and even numbers on the bottom row. They also completely skipped references to pins 1 and 26. Once these inconsistencies are noted, the pin-out is in accordance with that gleaned from the schematic for the Round-Cable Armadillo Interface and validation of the pin-to-pin connections in the cable connecting the Armadillo Interface to the TI-99/8.
As of September 2007, several additional schematic documents for the TI-99/8 have been added to the WHTech site. The new documents contain specifics for both versions of the TI-99/8 console and the internal logic diagrams for the DRAM controller (POLLO), HOMBRE, OSO, SKUNK, and VAQUERRO specialized chips used on both versions of the motherboard. The schematic for the Round-Cable Armadillo Interface card is also included.
Schematics for the 128K Memory expansion card, Flat-Cable Armadillo Interface, and the Round-Cable Armadillo Interface with Integrated 32K RAM are also available at WHTech as of June 2008, along with an extensive collection of technical data describing specific aspects of the internal operations of the TI-99/8. The documents include a complete functional description of the AMIGO chip, the final specialized chip used in the TI-99/8. A schematic for the AMIGO chip has not yet been located, however.
MESS can emulate a TI-99/8 with some limitations, given the appropriate ROMs. This makes the TI-99/8 (virtually) available to a wide audience, despite the rare occurance of the real machine.